1. Field of the Invention
The invention relates to a system for measuring the flow velocity of mud returning from the annulus of a borehole and, more particularly, the determination of the velocity of borehole fluid flow by the parametric generation of the difference frequency from combined, dual frequency acoustic signals.
2. History of the Prior Art
It has long been recognized in the oil industry that the collection of downhole data during drilling is of extreme value. Such information not only improves the efficiency of the drilling operation, but may serve as a warning to prevent dangerous situations from arising. A common, ever present danger in the drilling of a borehole is encountering an earth formation which contains high pressure fluids. When this occurs, the high pressure fluids from the formation enter the borehole and displace the drilling mud back up the borehole toward the drilling rig at the surface. If the intrusion of high pressure fluids back into the borehole is not detected quickly and controlled, it can result in the complete displacement of the drilling mud back up the borehole and expulsion of the high pressure fluids out of the top of the borehole. This event is called a "blow out" and can result in great injury to both property and life due to the high combustibility of the natural gas and other fluids and the violence with which they exit from the borehole.
On the other hand, it is possible that, during drilling, a borehole may enter a formation which is highly porous and create a tendency for all of the drilling mud to flow freely from the borehole into the porous formation. This event is termed "lost circulation" and can result in the substantial loss of drilling fluids if the lost circulation is not detected very rapidly and preventive measures taken. Upon the impendence of either of these two events, "blow-out" or "lost circulation", it is desirable to detect them as rapidly as possible in order to take remedial action to control the run-away mud flow and prevent either its substantial loss into a porous formation or to prevent its moving back up the borehole toward the surface and thereby prevent the possibility of personal injury and damage to equipment resulting from that rapid upward movement.
It is known to compare the input mud flow rate with the return mud flow rate in a borehole. A substantial increase in the rate of return mud flow with no corresponding increase in input flow is indicative of a "blow-out" whereas a substantial increase in input mud flow without a corresponding increase in the output flow is indicative of lost circulation. The biggest difficulty with prior art techniques for measuring these changes in mud flow rates has been that it has only been possible to make such measurements near the surface end of the borehole. Therefore in a deep borehole it was only possible to detect the imminence of "blow-out" at a location which is quite remote from the location down in the borehole where the event actually occurred. Thus, substantial amounts of time may have elapsed prior to the detection at the surface of the occurrence of an event downhole and substantial damage may have also occurred before remedial action could be taken.
The prior art is replete with techniques for monitoring the rate of mud flow returning from the annulus of a borehole. In recent years, systems which permit "measuring-while-drilling", which involves obtaining information continuously during drilling operations, have been favored. Among such systems, a number of different approaches exist for the critical step of continuously sensing and measuring the motion of the downhole fluids. A particular subset of these approaches has given rise to devices which sense the fluid motion by means of propagating acoustic energy through the fluid. In general, the physics of the measurement approach itself characterizes the different types of devices: Doppler shift, propagation time, and phase shift effects have each been used successfully to measure drilling fluid flow rates. Doppler devices utilize the frequency shift of the acoustic energy directed in the same direction as the flow relative to the acoustic energy directed in the opposite direction of the flow as the measure of fluid velocity. Such systems require the utilization of acoustic pulse generators which are operable in a downhole environment and which produce the necessary amplitude and frequency signal to overcome the technical problems associated with transmitting acoustic pulses in the hostile environment of a borehole.
A myriad of prior art problems plague flow velocity measurement devices which employ conventional acoustic pulse sensor systems. A primary source of concern is the generation of a "recognizable" acoustic signal without interfering with the flow of drilling fluid. The placement of a transmitter and a receiver directly in the borehole annulus effectively produces a recognizable signal but positioning a transducer structure in the path of fluid flow has serious and undesirable side effects. It is an advantage, therefore, to position the acoustic transmitter and receiver upon the conduit conducting the flow, however, this causes other problems. First, low frequency acoustic pulses are the ones which are most readily propagated in the drilling mud but the transducer hardware necessary to generate strong low frequency signals is generally too large for downhole applications. Secondly, the speed of sound in steel is approximately five times that in drilling fluid and the critical axial component of the acoustic energy is carried much more readily by the conduit than it is by the drilling mud. Higher frequency acoustic pulses will have smaller conduit components, but such signals have limited axial range because the cuttings entrained in the mud reflect and attenuate the signal. Often times the scattering loss is so severe with conventional ultrasonic flow measurement systems that the axial signal component is lost completely.
The system of the present invention overcomes many of the disadvantages of the prior art by making the measurement of downhole mud flow rates by low frequency acoustic signals propagated directly from within the fluid flow. The generation of such low frequency pulses was, heretofore, not generally thought to be feasible in downhole configurations due to the requisite size of the pulse generation unit.
Although more modern prior art approaches have included ultrasonic pulse generation systems which function with relatively small transducer structures, such ultrasonic flow devices do not provide satisfactory results in gas cut muds because the wall signals in the conduit greatly exceed the signals produced in the fluid. The present invention overcomes this particular difficulty by generating a low frequency pulse in the liquid where the two higher frequency, ultrasonic waves overlap. High frequency transducers having a frequency bandwidth typically of 500 KMz to 1 MHz are disposed upon the outside of the conduit to generate a strong, focused wave at the center of the fluid flow to be measured. The transmitter transducers are pulsed with a dual frequency wave of sufficient strength to generate nonlinear response of a non-gaseous liquid or to interact with existing or cavitated gas in the fluid, thereby generating a secondary (parametric) pulse of "difference frequency" waves in the fluid itself. The two different frequencies of the dual frequency signal are selected so that the secondary difference frequency pulse is at a low frequency having the appropriate attenuation characteristics and axial energy component to axially propagate to the receiver. A simple configuration has a pair of broadband transducers can be used, first transmitting and receiving a low frequency signal upstream and then downstream. The differences in sound speed can be used to derive flow velocity. In this manner, low frequency, high amplitude acoustic waves are generated directly in the mud with small transducers suitable for downhole use.